The present invention relates to semiconductor integrated circuit memories and, in particular, to input-output (I/O) buffers for controlling read and write operations.
Semiconductor integrated circuit memories include a plurality of memory elements or cells, which are arranged in rows and columns. The memory cells in each column are coupled to a respective pair of complementary bit lines. Each pair of bit lines has a differential voltage representing the data being read from or written to a memory cell in that column.
A typical memory I/O buffer includes a column multiplexer, a sense amplifier and a write driver. The column multiplexer multiplexes groups of bit lines into respective data input-output lines. There is a sense amplifier and a separate write driver for each pair of multiplexed data input-output lines. The write drivers and sense amplifiers are separate circuits that occupy separate physical areas on the integrated circuit in which the memory is fabricated. The separate circuits also require individual column multiplex select logic and control lines, one set for read accesses and one set for write accesses.
During a read access, the read column select logic for the selected column is enabled, and then the corresponding sense amplifier is enabled to sense the data provided by the enabled column. The write drivers are idle during a read access. During a write access, the write column select logic is enabled, and the write drivers amplify the input data onto the bit lines of the selected column. The sense amplifiers can be used or bypassed to sense the data being written through the write drivers.
The use of separate write drivers and sense amplifiers requires two large blocks of circuitry, which consumes a relatively large amount of physical area on the integrated circuit. This increases the cost of semiconductor memory dies and potentially limits the number of memory cells that can be fabricated on the die. Improved memory I/O buffers are therefore desired that use less area than a convention I/O buffer.